KR20200033291A - Manufacturing method of Ni-containing steel sheet - Google Patents

Abstract

The manufacturing method according to the embodiment of the present invention includes a predetermined chemical component composition, and after the hot rolling of the steel having the remaining Fe and unavoidable impurities, a quenching step of quenching from 800 ° C or more and 820 ° C or less, After maintaining at a heating temperature of 690 ° C or higher and 710 ° C or lower, an intermediate heat treatment process of cooling at an average cooling rate of 5 ° C / sec or higher to a cooling end temperature of 200 ° C or lower, and a tempering temperature of 570 ° C or higher and 600 ° C or lower The process is included in this order, and the parameter H prescribed | regulated by the heating temperature and holding time at intermediate heat processing, and the heating temperature and holding time at the time of tempering is set to 1.73x10 ^-6 or more and 1.96x10 ^-6 or less.

Description

Manufacturing method of Ni-containing steel sheet

The present disclosure relates to a method for manufacturing a Ni-containing steel sheet.

In recent years, the increase in global energy demand and the accompanying deterioration of the global environment have become a problem. Therefore, the demand for natural gas (LNG) as a clean energy source is rapidly increasing. With the increasing demand for natural gas (LNG), the construction of LNG storage tanks has been actively promoted at home and abroad in recent years. In such a situation, there is an increasing demand for a high-Ni steel plate (hereinafter sometimes simply referred to as a "steel plate") excellent in low-temperature toughness used for the main body of an LNG storage tank.

The high-Ni steel sheet is relatively inexpensive, and improves the toughness of the matrix by the addition of Ni, refinement of the structure by heat treatment, and the presence of residual austenite (hereinafter sometimes referred to as “residual γ”) that is stable even under cryogenic conditions. It is known to have excellent low-temperature toughness due to effects such as improvement of toughness by. Among high-Ni steel sheets, steel sheets with a Ni content of about 9% by mass (9% Ni steel) have been used as tank materials for LNG storage in 1963, and have been showing great results as tank materials, and the amount of usage will increase in the future. Is expected.

As described above, in the high-Ni steel sheet, low-temperature toughness is greatly improved by the presence of residual γ. However, when the steel sheet is processed and a large plastic strain is applied, the residual γ may cause a process organic transformation to martensite. When processed organic transformation occurs, the amount of residual γ decreases, and there is a possibility that low-temperature toughness deteriorates.

In such a situation, various techniques have not been studied for deteriorating low-temperature toughness in high Ni steel sheets even when large plastic deformation is imparted.

For example, Patent Document 1 discloses a method of manufacturing a steel sheet of Ni-containing steel having sufficient low-temperature toughness even when an extremely thick material having a plate thickness exceeding 40 mm is used. In Patent Document 1, as the Ni-containing steel as a material, C, Ni, and Mn are contained in a predetermined range, and P and S in impurities are each suppressed extremely low to 0.001% by weight (mass)% or less. Then, after the steel is hot rolled, two quenching and quenching treatments are performed under specific conditions. Thereby, low-temperature toughness is improved.

Moreover, in patent document 1, in FIG. 2, the aging treatment of 250 ° C. × 1 hour was performed after additionally giving 5% of tensile pre-strain after the above heat treatment, and the rolling direction at -196 ° C. (L direction) And the result of examining the Charpy impact energy in a right angle direction (C direction). In Patent Document 1, according to FIG. 2, it is described that the low-temperature toughness of the steel sheet itself and the welded seam has been dramatically improved by setting P to 0.001% by weight (mass) or less.

Japanese Patent Publication No. Hei 6-179909

In patent document 1, it is necessary to regulate the upper limit of P content to 0.001 mass%. However, if the cleanliness is increased in order to set the upper limit of the P content to 0.001 mass%, there is a problem that productivity deteriorates.

The embodiment of the present invention has been made in light of such a situation, and its object is to provide a method for producing a Ni-containing steel sheet excellent in low-temperature toughness after plastic deformation is applied even when the P content exceeds 0.001% by mass.

Aspect 1 of the embodiment of the present invention,

C: 0.040 mass% or more and 0.060 mass% or less,

Si: 0.10 mass% or more and 0.30 mass% or less,

Mn: 0.50 mass% or more and 0.70 mass% or less,

P: 0.0010 mass% or more and 0.0025 mass% or less,

S: 0.0010 mass% or less,

Ni: 9.10 mass% or more and 9.40 mass% or less,

Al: 0.020 mass% or more and 0.050 mass% or less, and

N: 0.0050 mass% or less

And after the hot rolling of the steel, the balance of which is Fe and inevitable impurities,

Quenching process for quenching from a quenching temperature of 800 ° C or higher and 820 ° C or lower,

After maintaining at a heating temperature of 690 ° C or more and 710 ° C or less, an intermediate heat treatment process for cooling at an average cooling rate of 5 ° C / sec or more to a cooling end temperature of 200 ° C or less, and

A tempering process of tempering at a tempering temperature of 570 ° C or higher and 600 ° C or lower

In this order,

In the intermediate heat treatment step and the tempering step, it is a method for producing a Ni-containing steel sheet in which the parameter H represented by the following formula (1) is 1.73 × 10 ^-6 or more and 1.96 × 10 ^-6 or less.

H = {(D _{Ni, L} × t _L ) ^0.5 + (D _{Ni, T} × t _T ) ^0.5 } × [Ni] + {(D _{C, L} × t _L ) ^0.5 + (D _{C, T} × t _T ) ^0.5 } × [C] ··· (1)

From here,

t _L : Heating holding time in the middle heat treatment process (seconds)

t _T : Heating and holding time (seconds) in the tempering process

[Ni]: Ni content (% by mass)

[C]: C content (% by mass)

D _{Ni, L} = 1.4 × 10 ^-4 × exp (-29.58 × 1000 / T _L )

D _{Ni, T} = 1.4 × 10 ^-4 × exp (-29.58 × 1000 / T _T )

D _{C, L} = 0.45 × 10 ^-4 × exp (-18.54 × 1000 / T _L )

D _{C, T} = 0.45 × 10 ^-4 × exp (-18.54 × 1000 / T _T )

Meanwhile,

T _L : Heating temperature (K) in the intermediate heat treatment process

T _T : tempering temperature (K)

Aspect 2 of the embodiment of the present invention,

The said steel is the manufacturing method in aspect 1 which is any one or more of the following (a)-(d).

(a) The content of C is 0.045 mass% or more and 0.060 mass% or less,

(b) The Si content is 0.15 mass% or more and 0.30 mass% or less,

(c) The content of the Mn is 0.60% by mass or more and 0.70% by mass or less, and

(d) The Al content is 0.020 mass% or more and 0.045 mass% or less

Aspect 3 of the embodiment of the present invention,

The steel is further Cu: 0.01% by mass or more and 0.20% by mass or less, Cr: 0.01% by mass or more and 0.20% by mass or less, Mo: 0.01% by mass or more and 0.20% by mass or less, V: 0.1% by mass or less, Nb: 0.1% by mass % Or less, Ti: 0.1% by mass or less, and B: 0.005% by mass or less.

According to the embodiment of the present invention, even when the P content exceeds 0.001% by mass, a Ni-containing steel sheet excellent in low-temperature toughness after plastic deformation can be produced.

[Fig. 1] Fig. 1 is a diagram showing the relationship between the parameter H and the brittle fracture rate after applying plastic deformation in the embodiment of the present invention.

As a result of earnest examination, the present inventors control the parameter H defined by the heating temperature and holding time during intermediate heat treatment, and the heating temperature and holding time during tempering to a predetermined range, whereby the P content exceeds 0.001% by mass. It was also found that a steel sheet excellent in low-temperature toughness (hereinafter sometimes referred to as "deformed aging characteristic") after plastic deformation is applied can be produced.

1 is a view showing the relationship between the parameter H and the brittle fracture rate after plastic deformation is applied, which is an index of strain aging characteristics. As shown in Fig. 1, the present inventor can make the brittle fracture rate after giving plastic deformation less than or equal to 5% by setting the parameter H to 1.73 x 10 ^-6 or more and 1.96 x 10 ^-6 or less, and the deformation aging characteristics It has been found that this excellent steel sheet can be produced.

By controlling the parameter H, the details of the mechanism by which the strain aging characteristics are improved even when the P content exceeds 0.001 mass% are unknown. However, at the present time, the inventor considers the mechanism as follows.

When the P content is increased, P is segregated much in the austenite grain boundaries, and the grain boundaries are generally embrittled. For this reason, when the P content increases, the strain aging characteristics may deteriorate.

In the embodiment of the present invention, the diffusion of C and Ni in the metal structure is controlled by controlling the parameter H (that is, controlling the heating temperature and the holding time during intermediate heat treatment, and the heating temperature and the holding time during tempering). Control. Specifically, if the parameter H is increased, diffusion of C and Ni at the time of intermediate heat treatment and tempering is promoted, and when the parameter H is decreased, diffusion of C and Ni at the time of intermediate heat treatment and tempering is suppressed. do. In the embodiment of the present invention, the parameter H is controlled within a predetermined range to control the diffusion of C and Ni at the time of intermediate heat treatment and tempering, and consequently the concentration of C and Ni to residual γ.

Here, the stability of residual γ, in which residual γ remains without processing organic transformation even when plastic deformation is applied to the steel sheet, greatly contributes to concentration of C and Ni into residual γ. Therefore, in the embodiment of the present invention in which the concentration of C and Ni to the residual γ is appropriately controlled, the deterioration of the strain aging characteristics due to the increase in the P content is compensated, and a steel sheet having excellent strain aging characteristics can be produced. I think I can.

1. Chemical composition

Hereinafter, the chemical composition of the steel sheet produced in the embodiment of the present invention will be described.

In the following description, basic elements, C, Si, Mn, P, S, Ni, Al, and N are first described, and elements that may be selectively added are described.

[C: 0.040 mass% or more and 0.060 mass% or less]

C is an element that increases the strength of the steel sheet, and in order to secure a desired high strength, it is necessary to contain 0.040% by mass or more. On the other hand, the content exceeding 0.060 mass% causes a decrease in low-temperature toughness. For this reason, C content is made into 0.040 mass% or more and 0.060 mass% or less. The lower limit of the C content is preferably 0.045% by mass in order to further contribute to the increase in strength.

[Si: 0.10 mass% or more and 0.30 mass% or less]

Si acts as a deoxidizer and is an element that improves the strength of steel, and in order to obtain such an effect, it needs to contain 0.10 mass% or more. On the other hand, when it is contained in a large amount in excess of 0.30% by mass, the embrittlement embrittlement sensitivity becomes high. For this reason, Si content was made into 0.10 mass% or more and 0.30 mass% or less. The lower limit of the Si content is preferably 0.15% by mass in order to further contribute to the increase in strength.

[Mn: 0.50 mass% or more and 0.70 mass% or less]

Mn needs to add 0.50 mass% or more in order to contribute to the increase in strength. On the other hand, when Mn is added in excess of 0.70% by mass, the embrittlement susceptibility is increased, and toughness is reduced. For this reason, Mn content is made into 0.50 mass% or more and 0.70 mass% or less. The lower limit of the Mn content is preferably 0.60% by mass in order to further contribute to the increase in strength.

[P: 0.0010 mass% or more and 0.0025 mass% or less, S: 0.0010 mass% or less]

Since P and S are both elements that decrease toughness, it is preferable to reduce them as much as possible, but it is acceptable in the range of 0.0025 mass% or less and 0.0010 mass% or less (not including 0 mass%), respectively.

About P, it is added 0.0010 mass% or more and 0.0025 mass% or less from an economical viewpoint. In consideration of economic efficiency, it is more preferably 0.0015 mass% or more and 0.0025 mass% or less.

[Ni: 9.10 mass% or more and 9.40 mass% or less]

Ni is an essential element in the embodiment of the present invention, and has the effect of imparting high toughness to the steel sheet at low temperatures, but less than 9.10% by mass, the effect is insufficient. On the other hand, even if it is added in a large amount in excess of 9.40% by mass, the effect reaches saturation and is not economical. For this reason, Ni content was made into 9.10 mass% or more and 9.40 mass% or less.

[Al: 0.020 mass% or more and 0.050 mass% or less]

Al needs to be added in an amount of 0.0020% by mass or more as a deoxidizer, but if it is added in excess of 0.050% by mass, cleanliness is lowered. Therefore, the Al content is set to be 0.020 mass% or more and 0.050 mass% or less. The upper limit of the Al content is preferably 0.045% by mass in order to further increase the cleanliness.

[N: 0.0050% by mass or less]

N decreases toughness in the solid solution state, but also has the effect of becoming AlN to refine the crystal grains. Therefore, N is reduced as much as possible within a range in which the crystal grains are not coarsened. For this reason, N was set to 0.0050 mass% or less (not including 0 mass%).

[Remainder]

In one preferred embodiment, the remainder is iron and unavoidable impurities. As an unavoidable impurity, mixing of trace elements (eg, As, Sb, Sn, etc.) brought in by circumstances such as raw materials, materials, and manufacturing facilities is permitted. On the other hand, as for example, P and S, usually, the smaller the content, the more preferable, and thus an inevitable impurity, but there are elements that are separately specified as described above for the composition range. For this reason, in this specification, when it is called "the unavoidable impurity" which comprises a remainder, it is a concept except the element whose composition range is separately defined.

However, it is not limited to this embodiment. As long as the characteristics of the steel sheet produced by the manufacturing method according to the embodiment of the present invention can be maintained, any other elements may be further included. Other elements which can be selectively contained as described above are exemplified below.

[Cu: 0.01% by mass or more and 0.20% by mass or less, Cr: 0.01% by mass or more and 0.20% by mass or less, Mo: 0.01% by mass or more and 0.20% by mass or less, V: 0.1% by mass or less, Nb: 0.1% by mass or less, Ti: 0.1% by mass or less and B: 0.005% by mass or less;

Cu, Cr, Mo, V, Nb, Ti, and B are elements contributing to strength improvement, and may be selected and contained in one or more kinds as necessary. In order to contribute to improving the strength, it is preferable to add Cu at least 0.01 mass%, Cr at least 0.01 mass%, and Mo at 0.01 mass% or more. On the other hand, since it causes the toughness of the base material to decrease, Cu is 0.20 mass% or less, Cr is 0.20 mass% or less, Mo is 0.20 mass% or less, V is 0.1 mass% or less, Nb is 0.1 mass% or less, and Ti is 0.1 It is preferable to add 0.005 mass% or less of mass% or less and B.

2. Manufacturing method

Next, a manufacturing method according to an embodiment of the present invention will be described.

In the following description of the manufacturing method, a mechanism capable of obtaining a desired metal structure by such a manufacturing method and improving various characteristics may be described. These are the mechanisms considered by the knowledge obtained by the present inventors at the present time, but it is intended to be noted that the technical scope of the present invention is not limited.

The present inventors quenching a rolled material having a predetermined chemical composition from a predetermined quenching temperature, and performing heat treatment and tempering by strictly controlling the heating time and the holding time so that the parameter H to be described later becomes a predetermined range. , It has been found that even when the P content exceeds 0.001% by mass, a steel sheet having excellent strain aging characteristics can be manufactured.

The details will be described below.

First, by the conventional method, it is preferable to melt the steel-making raw material satisfying the requirements of the chemical composition with a conventional solvent such as a converter and to make a slab (material steel) by a continuous casting method. The obtained material steel is heated to a temperature that can be hot rolled by a conventional method, followed by hot rolling (AR: As-Roll) to obtain a steel sheet having a desired plate thickness (for example, 32 mm).

[Quenching process (quenching temperature: 800 ° C or more and 820 ° C or less)]

Subsequently, in order to obtain a uniform martensite structure, it is quenched after reheating to a quenching temperature of 800 ° C or higher and 820 ° C or lower, and a quenching treatment is performed. It is preferable to perform quenching at an average cooling rate of 5 ° C / sec or higher to a cooling end temperature of 200 ° C or lower. Rapid cooling is performed, for example, by water cooling or the like. For example, if it is water cooling, the average cooling rate is 5 ° C / sec or more to a cooling end temperature of 200 ° C or less. When the quenching temperature exceeds 820 ° C, the austenite grains are coarsened by recrystallization, and as a result, the low-temperature toughness of the steel sheet may deteriorate. On the other hand, if the quenching temperature is less than 800 ° C, quenching becomes insufficient, causing deterioration of strain aging characteristics, and the strength of the steel sheet may be insufficient.

[Intermediate heat treatment process (after maintaining at a heating temperature of 690 ° C or more and 710 ° C or less, cooling to an average cooling rate of 5 ° C / sec or more to a cooling end temperature of 200 ° C or less)]

Subsequently, the mixture is reheated to a heating temperature (intermediate heat treatment temperature) of 690 ° C or higher and 710 ° C or lower, which is a two-phase region in which ferrite and austenite coexist, and is cooled after being held for a predetermined time after reaching the heating temperature. Cooling is performed at an average cooling rate of 5 ° C / sec or higher to a cooling end temperature of 200 ° C or lower. Cooling is performed, for example, by water cooling. For example, if it is water cooling, the average cooling rate is 5 ° C / sec or more to a cooling end temperature of 200 ° C or less.

The uniform martensite structure obtained by the above-mentioned quenching process is transformed into a ferrite structure and an austenite structure when heated to a heating temperature in the two-phase region. Then, C and Ni diffuse through the austenite structure through the process of being heated and maintained, and as a result, C and Ni are concentrated in the austenite structure. Thereafter, by quenching, the austenite structure is transformed into a martensite structure, and a mixed ferrite structure and a martensitic structure in which C and Ni are concentrated are generated.

If the intermediate heat treatment temperature is less than 690 ° C, the amount of austenite produced in the tempering process, which is the next step, is insufficient, resulting in deterioration of strain aging characteristics. On the other hand, when the intermediate heat treatment temperature exceeds 710 ° C., the single-phase region temperature ranges, and since a ferrite structure is not generated, an austenite structure in which C and Ni are concentrated cannot be obtained. As a result, austenite is not generated in the tempering step, which is the next step, resulting in deterioration of strain aging characteristics.

If the cooling end temperature is more than 200 ° C or the average cooling rate is less than 5 ° C / sec, a martensite structure is not obtained.

[Polishing process (polishing temperature: 570 ° C to 600 ° C)]

Subsequently, reheating is performed at a tempering temperature of 570 ° C or higher and 600 ° C or lower, and a tempering treatment is performed for holding for a predetermined time after reaching the temperature. The cooling method is not particularly limited, but it is preferably, for example, air cooling.

When the ferrite structure obtained by the intermediate heat treatment process described above and the martensite structure in which C and Ni are concentrated are polished, a part of the martensite structure is transformed into an austenite structure. This reverse-transformed austenite structure becomes residual austenite. In more detail, even in the martensitic structure obtained by the intermediate heat treatment step, there are portions in which C and Ni are concentrated heavily and portions in which C and Ni are not very concentrated. When the martensitic structure is tempered, the portion where the C and Ni are concentrated strongly is lowered by the A _s point (the initiation temperature of reverse transformation), and thus, it is reverse-transformed into an austenite structure even at a temperature around the tempering temperature. In this reverse-transformed austenite structure, C and Ni are concentrated heavily. On the other hand, in the portion where C and Ni are not very concentrated, since the point of A _s is not lowered so much, inverse transformation does not occur, and normal polishing treatment for adjusting hardness and the like is performed.

As described above, the final metal structure after the tempering process includes a ferrite structure, a martensite structure, and a residual γ structure. On the other hand, it is thought that C and Ni are further concentrated by heating and holding the austenite structure heated to a tempered temperature and reverse-transformed. Thus, C and Ni are concentrated in the residual γ obtained in the embodiment of the present invention. For this reason, the strain aging characteristics of the steel sheet obtained by the manufacturing method according to the embodiment of the present invention are improved.

When the tempering temperature is less than 570 ° C, the amount of residual γ in the obtained steel sheet is small, resulting in deterioration of strain aging characteristics. On the other hand, when the tempering temperature exceeds 600 ° C, both the size and amount of residual γ increase, resulting in deterioration of the strain aging characteristics. Also, from the viewpoint of securing the strength of the steel sheet, a tempering temperature exceeding 600 ° C is not preferable.

[Parameter H: 1.73 × 10 ^-6 or more and 1.96 × 10 ^-6 or less]

In the embodiment of the present invention, in order to improve the strain aging characteristics, in the intermediate heat treatment step and the tempering step described above, the parameter H represented by the following formula (1) is 1.73 × 10 ^-6 or more and 1.96 × 10 ^-6 or less Shall be

H = {(D _{Ni, L} × t _L ) ^0.5 + (D _{Ni, T} × t _T ) ^0.5 } × [Ni] + {(D _{C, L} × t _L ) ^0.5 + (D _{C, T} × t _T ) ^0.5 } × [C] ··· (1)

From here,

t _L : Heating holding time in the middle heat treatment process (seconds)

t _T : Heating and holding time (seconds) in the tempering process

[Ni]: Ni content (% by mass)

[C]: C content (% by mass)

D _{Ni, L} = 1.4 × 10 ^-4 × exp (-29.58 × 1000 / T _L )

D _{Ni, T} = 1.4 × 10 ^-4 × exp (-29.58 × 1000 / T _T )

D _{C, L} = 0.45 × 10 ^-4 × exp (-18.54 × 1000 / T _L )

D _{C, T} = 0.45 × 10 ^-4 × exp (-18.54 × 1000 / T _T )

Meanwhile,

T _L : Heating temperature (K) in the intermediate heat treatment process

T _T : tempering temperature (K)

In order to improve the strain aging characteristics, it is important to improve the stability of the residual γ so that residual γ is generated in the steel sheet and the residual γ does not undergo organic transformation. In order to generate residual γ in the steel sheet, it is important to concentrate C and Ni in the austenite structure during intermediate heat treatment. In addition, in order to improve the stability of residual γ, it is important to appropriately control the concentration of C and Ni to residual γ. As will be described later, when C and Ni are excessively concentrated in the residual γ, the strain aging characteristics may deteriorate. Thus, the concentration of C and Ni into the austenite structure greatly contributes to both the formation of residual γ and the stability of residual γ. In addition, the diffusion of C and Ni is related to the concentration of C and Ni into austenite. Therefore, in the embodiment of the present invention, attention was paid to the diffusion of C and Ni.

Elemental diffusion is basically proportional to the square root of the product of the diffusion coefficient and time. Therefore, the square root of this product was calculated | required for each element of C and Ni, and the formula which added them was defined as parameter H. In addition, when the square root of the product of the diffusion coefficient and time is obtained for each element of C and Ni, the parameter H is defined so as to consider each heat treatment of intermediate heat treatment and tempering. The parameter H defined as described above is an index indicating the degree of diffusion of C and Ni at the time of intermediate heat treatment and tempering. If the parameter H is less than 1.73 x 10 ^-6 , the diffusion of C and Ni into the austenite structure at the time of intermediate heat treatment is insufficient, and the amount of residual γ in the steel sheet is insufficient, thereby deteriorating the strain aging characteristics. On the other hand, when the parameter H exceeds 1.96 × 10 ⁻⁶ , C and Ni are excessively diffused into the austenite structure, and the amount of residual γ decreases, thereby deteriorating the strain aging characteristics.

Example

1. Sample making

The steel sheet having the chemical composition shown in Table 1 was manufactured by hot rolling the cast steel and subjecting the obtained steel piece to heat treatment shown in Table 2. The plate thickness of the produced steel sheets was all 32 mm. Then, samples were taken from these steel sheets. On the other hand, cooling at the time of quenching treatment and intermediate heat treatment was performed by water cooling.

In addition, in Table 2, the underlined numerical value indicates that it is outside the range of the embodiment of the present invention.

2. Characteristic evaluation

Next, various characteristics were evaluated under the conditions shown below.

[Tensile test]

From the t / 4 position (t: plate thickness) of each steel sheet, a JIS 4 tensile test piece was taken from the steel sheet so that the direction perpendicular to the rolling direction of the steel sheet was a long direction, and according to the method specified in JIS Z2241: 2011, Yield strength and tensile strength were measured. Table 3 shows the results.

[Charpy impact test after plastic deformation is given]

After 5% of plastic deformation was applied to each steel sheet, aging treatment was performed at 250 ° C for 1 hour. Next, three Charpy impact test pieces (V notched test piece of JIS Z2242: 2005) were taken from the t / 4 position (t: plate thickness) of each steel sheet so that the direction perpendicular to the rolling direction of the steel sheet was a long direction. Then, the brittle fracture rate (%) at -196 ° C was measured by the method described in JIS Z2242: 2005, and all three specimens were said to have excellent strain aging characteristics for samples having a brittle fracture rate of 5% or less. On the other hand, Table 3 shows three measured values measured using three test pieces.

Consider the results in Table 3.

Sample No. 1 to 5 and 15 are samples produced by a production method that satisfies the requirements of the embodiments of the present invention, and all three test pieces have a brittle fracture rate of 5% or less, and thus have excellent strain aging characteristics.

Meanwhile, sample No. All of 1-5 and 15 were excellent in yield strength and tensile strength, and were high strength.

Meanwhile, sample No. 6 to 14 are samples produced by a manufacturing method that does not satisfy the requirements of the embodiments of the present invention, and the brittle fracture rate of at least one of the three test pieces exceeds 5%, resulting in poor deformation aging characteristics. there was.

Sample No. 6 had a low tempering temperature and a parameter H, so the strain aging characteristics were inferior.

Sample No. Since the parameter H was high in 7 to 9, the strain aging characteristics were inferior.

Sample No. 10 had low intermediate heat treatment temperature and parameter H, so the strain aging characteristics were inferior.

Sample No. 11 had a high parameter H, so the strain aging characteristics were inferior.

Sample No. 12, the intermediate heat treatment temperature and the parameter H were high, so the strain aging characteristics were inferior.

Sample No. Since the tempering temperature and the parameter H were high in 13, the strain aging characteristics were inferior.

Sample No. Since the quenching temperature of 14 was low, the deformation aging characteristics were inferior.

This application claims priority claims based on Japanese Patent Application No. 2017-162740 with the filing date of August 25, 2017, and Japanese Patent Application No. 2018-131749 with the filing date of July 11, 2018. Entails. No. 2017-162740 and No. 2018-131749 are incorporated herein by reference.

Claims (3)

C: 0.040 mass% or more and 0.060 mass% or less,
Si: 0.10 mass% or more and 0.30 mass% or less,
Mn: 0.50 mass% or more and 0.70 mass% or less,
P: 0.0010 mass% or more and 0.0025 mass% or less,
S: 0.0010 mass% or less,
Ni: 9.10 mass% or more and 9.40 mass% or less,
Al: 0.020 mass% or more and 0.050 mass% or less, and
N: 0.0050 mass% or less
And after the hot rolling of the steel, the balance of which is Fe and inevitable impurities,
Quenching process for quenching from a quenching temperature of 800 ° C or higher and 820 ° C or lower,
After maintaining at a heating temperature of 690 ° C or more and 710 ° C or less, an intermediate heat treatment process for cooling at an average cooling rate of 5 ° C / sec or more to a cooling end temperature of 200 ° C or less, and
A tempering process of tempering at a tempering temperature of 570 ° C or higher and 600 ° C or lower
In this order,
In the intermediate heat treatment step and the tempering step, the method H for producing a Ni-containing steel sheet, wherein the parameter H represented by the following formula (1) is 1.73 × 10 ^-6 or more and 1.96 × 10 ^-6 or less.
H = {(D _{Ni, L} × t _L ) ^0.5 + (D _{Ni, T} × t _T ) ^0.5 } × [Ni] + {(D _{C, L} × t _L ) ^0.5 + (D _{C, T} × t _T ) ^0.5 } × [C] ··· (1)
From here,
t _L : Heating holding time in the middle heat treatment process (seconds)
t _T : Heating and holding time (seconds) in the tempering process
[Ni]: Ni content (% by mass)
[C]: C content (% by mass)
D _{Ni, L} = 1.4 × 10 ^-4 × exp (-29.58 × 1000 / T _L )
D _{Ni, T} = 1.4 × 10 ^-4 × exp (-29.58 × 1000 / T _T )
D _{C, L} = 0.45 × 10 ^-4 × exp (-18.54 × 1000 / T _L )
D _{C, T} = 0.45 × 10 ^-4 × exp (-18.54 × 1000 / T _T )
Meanwhile,
T _L : Heating temperature (K) in the intermediate heat treatment process
T _T : tempering temperature (K) According to claim 1,
The said steel is the manufacturing method of any one or more of the following (a)-(d).
(a) The content of C is 0.045 mass% or more and 0.060 mass% or less,
(b) The Si content is 0.15 mass% or more and 0.30 mass% or less,
(c) The content of the Mn is 0.60% by mass or more and 0.70% by mass or less, and
(d) The Al content is 0.020 mass% or more and 0.045 mass% or less The method of claim 1 or 2,
The steel is further Cu: 0.01% by mass or more and 0.20% by mass or less, Cr: 0.01% by mass or more and 0.20% by mass or less, Mo: 0.01% by mass or more and 0.20% by mass or less, V: 0.1% by mass or less, Nb: 0.1% by mass % Or less, Ti: 0.1 mass% or less, and B: 0.005 mass% or less.

Images